This invention relates to a transmitting apparatus in a ring network and, more particularly, to a transmitting apparatus in a ring network in which a plurality of transmitting apparatus are connected in ring form so as to be capable of transmitting in each of upstream and downstream directions, working and protection channels are assigned to each direction and, when failure occurs in a transmission path, a transmit signal is looped back using the protection channel to effect rescue.
Frame Structure
Synchronous optical networks (SONET), which utilize optical communication that is capable of high-capacity transmission, have become widespread owing to an increase in communication traffic. With SONET, user data undergoes multiplexed transmission in accordance with a Synchronous Transport Signal (STS-N) frame (where N represents an integer) format. FIG. 23A is a diagram showing the structure of a 51.84-Mbps STS-1 frame. The frame has 9×90 bytes overall (810 bytes/125 μs), of which 3×9 bytes constitute overhead OH and 87×9 bytes constitute an STS payload STS-1 SPE (Synchronous Payload Envelope). Nine bytes of the payload constitute path overhead POH, and packets (VT packets) of multiple lower-order channels are multiplexed onto the remaining 86×9 bytes. With SONET, frame formats other than the STS-1 frame format mentioned above include STS-3 (155.52 Mbps), STS-12 (622.08 Mbps) and STS-48 (2.488 Gbps). These frame formats can be used in appropriate fashion by optical transmission lines.
VT (Virtual Tributary) packets are of various types, such as VT 1.5, VT 2, VT 3 and VT 6. A VT 1.5 packet is composed of 27 (=3×9) bytes, as shown in FIG. 23B, and the bit rate of one VT channel is 1.728 Mbps (=27×8/125 Mbps). FIG. 23C is a diagram useful in describing mapping of VT 1.5 packets into VT-structured STS-1 SPE. A first column is for path overhead POH, and 30th and 59th columns are for fixed stuff composed of all “1”s. As a result, the STS-1 SPE is divided into three areas of 28 columns each. The 1st to 28th columns of each area are assigned 1-1, 2-1, 3-1, 4-1, 5-1, 6-1, 7-1, 1-2, 2-2, . . . , 7-4 sequentially. VT 1.5 packets of the first channel are placed in the 2nd, 31st and 60th rows, VT 1.5 packets of the second channel are placed in the 3rd, 32nd and 61th rows, and VT 1.5 packets of the 28th channel are placed in the 29th, 58th and 87th rows.
Ring Structure
A ring structure in which a transmitting apparatus is connected in the form of a ring from the viewpoint of assuring reliability is known as a network configuration for SONET. The ring structure is such that if a failure occurs in a transmission path, the transmission can be continued via an alternative transmission path, thereby making it possible to improve the reliability of transmission. FIG. 24 is a block diagram illustrating the structure of an ADM (Add/Drop Mux) transmitting apparatus that can be ring-connected. FIG. 25 is a diagram useful in describing the ring structure.
The ADM transmitting apparatus is terminal equipment having a MUX (multiplexing) function and an add/drop function. More specifically, the apparatus has a cross-connect function and an add/drop function for the lower-order side (tributary side). Line interfaces (LINE IF) 1a, 1b receive higher-order signals (e.g., OC-48: 2.488-Gbps optical signals) from optical transmission lines 8a1, 8b1 on WEST and EAST sides, respectively, convert these signals to electrical signals and execute processing based upon overhead information. Demultiplexers (DMUX) 2a, 2b demultiplex higher-order signals into lower-order signals (e.g., STS-1 electrical signals), a cross-connect unit 3 performs cross connect on the STS level, multiplexers (MUX) 4a, 4b multiplex the cross-connected STS-1 signals into higher-order signals and line interfaces (LINE IF) 5a, 5b add overhead onto these higher-order signals, convert the signals to optical signals and send the optical signals to optical transmission lines 8a2, 8b2 on the EAST and WEST sides, respectively. It should be noted that a signal direction in which a signal is input to the EAST side of the transmitting apparatus (node) and output from the WEST side shall be referred to as the EW (EAST→WEST) direction, and that a signal direction in which a signal is input to the WEST side of the transmitting apparatus (node) and output from the EAST side shall be referred to as the WE (WEST→EAST) direction.
The STS/VT cross-connect unit 3 switches, on the STS level, STS-1 signals inserted from tributary interfaces 6a, 6b, . . . via MUX/DMUXs 7a, 7b, . . . and sends these switched signals in the WE or EW direction. The STS/VT cross-connect unit 3 also drops signals, which have been received from the transmission path from the WE or EW direction, on the tributary side, demultiplexes these signals to lower-order signals of a prescribed speed via the MUX/DMUXs 7a, 7b, . . . and sends the signals to the tributary side from the tributary interfaces 6a, 6b, . . . The transmission paths in the WE and EW directions both have working and protection channels assigned to them. For example, in case of OC-48, 1st to 24th channels of the 48 STS-1 channels are working channels and 25th to 48th channels are protection channels. The transmitting apparatus normally transmits signals using the working channel. When a failure occurs, rescue is performed using a protection channel.
Protection at Time of Transmission-path Failure
In accordance with the ring architecture, ADM transmitters 10a to 10d are connected in the form of a ring, as shown in FIG. 25. If a certain transmission path develops a failure or suffers a decline in quality, signals are transmitted in a direction that avoids this transmission path, thereby allowing communication to continue and assuring reliability and quality. Networks in which multiple nodes have been connected into a ring can be classified broadly into two types of schemes, namely a UPSR (Uni-directional Path Switched Ring) scheme and a BLSR (Bi-directional Line Switched Ring) scheme. In comparison with the UPSR scheme, the BLSR scheme is advantageous in that channel capacity can be enlarged because the same channel can be used between different nodes (spans).
The BLSR scheme is such that if failures occur at a plurality of locations and sever a ring transmission path, a signal that cannot reach a destination node may be produced and the signal may be transmitted to another node by loop-back for rescue purposes. In order to prevent such misconnection, squelch is performed. In the squelch operation, a P-AIS (Path Alarm Indication Signal) is transmitted upon inserting this signal in the signal of the channel that cannot reach the target node.
FIGS. 26A, 26B are diagrams useful in describing rescue from failure. With the UPSR scheme, as shown in FIG. 26A, the same signal is sent in the EW direction from a node (C) to a node (B) and in the WE direction from the node (C) to a node (D) by, e.g., a channel ch.1, and a node (A) selects and receives the signal of channel ch. 1 by a path switch PathSW. Accordingly, even if a failure develops between nodes (A) and (B), as shown in FIG. 26B, node (A) is capable of selecting and receiving the signal on channel ch. 1 via node (D) by the path switch PathSW, thereby allowing communication between nodes C and A to continue.
With the BLSR scheme, as shown in FIG. 26C, the node (C) sends a signal to node (A) by channel ch. 1 in, e.g., the EW direction and sends the signal to node (D) by channel ch. 1 in the WE direction, and node (D) sends the signal to node (A) by channel ch. 1 in the WE direction (the same channel is used in a different span). In other words, communication is possible between nodes (C) and (A), between nodes (C) and (D) and between nodes (D) and (A) using the same channel ch. 1. Channel capacity, therefore, can be enlarged as compared with the UPSR scheme.
The BLSR scheme is such that if a failure develops between nodes (A) and (B), as depicted in FIG. 26D, rescue is performed by an ASP (Automatic Protection Switch) protocol using K1, K2 bytes. With the APS protocol, the signal sent from node (C) to protection channel ch.1 is looped back to protection channel ch. 25 at node (B), and the protection channel ch. 25 is switched over to working channel ch. 1 at node (A), whereby communication between nodes (C) and (A) is allowed to continue. It should be noted that communication between nodes (C) and (D) is performed on channel ch. 1 and that communication between nodes (D) and (A) also is performed on ch. 1 because such communication does not traverse the faulty segment.
FIGS. 27 to 30 are diagrams useful in describing the APS protocol, in which WK represents a working channel and PT a protection channel (indicated by hatching. Nodes (A) to (H) are connected in a ring configuration by different transmission paths in each of WE and EW directions, and a working channel and protection channel are assigned to each transmission path.
FIG. 27 illustrates a case where communication is performed bi-directionally between nodes (A) and (E). If under these circumstances a failure occurs between nodes (F) and (E) in the transmission path in the EW direction, as shown in FIG. 28, node (E) detects the failure, becomes a switching node and sends the opposing node (F) switching requests (SF-RING; Signal Failure Ring) 51, 52, which indicate transmission-path failure, in both of short-path and long-path directions, respectively, in accordance with the APS protocol. If, upon receiving the switching requests, the nodes (D), (C), (B), (A), (H) and (G) recognize that the destination of request 52 is node (F) and not these nodes themselves, a state of full pass-through is established and the signal is allowed to pass through the protection channel. Upon receiving the request 51 on the short path, node (F) becomes a switching node, sends a reverse request (RR-RING; Reverse Request Ring) over the short path and sends a request 53 (SF-RING), which is identical with the received request 52, over the long path.
In the event of a failure, bridging and switching are executed simultaneously at reception of the request from the long path. Bridging represents a state in which the same traffic is sent by being switched from a working channel to a protection channel, and switching represents a state in which traffic from a protection channel is sent upon being switched to a working channel. Accordingly, owing to occurrence of the failure between nodes (F) and (E), node (E) forms a bridge and sends the signal destined for node (A) to the protection channel PT, as indicated by the dashed line in FIG. 29, and node (F) forms a switch for switching the protection channel PT to the working channel WK from node (F) in the direction toward node (A), as indicated by the dashed line in FIG. 29. The foregoing illustrates rescue of a signal from node (E) to node (A), though a signal from node (A) to node (E) can be rescued in a similar manner. More specifically, as shown in FIG. 30, in this case node (F) forms a bridge for looping back a signal, which was directed from node (A) to node (E) over the working channel, to the protection channel PT, and node (E) performs switching to switch from the protection channel PT to the working channel. Communication between nodes (E) and (A) can therefore continue.
The bytes K1, K2 used in the APS protocol are contained in the section overhead SOH, as shown in FIG. 31. The K1 byte comprises a switching request of 1st to 4th bits and a remote office ID (the identification number of the node that is the destination of the K1 byte) of 5th to 8th bits, and the K2 byte comprises a local office ID (the identification number of the node generating the request) of 1st to 4th bits, a 5th bit (S/L bit) indicating whether the request is a short-path request (“0”) or a long-path request (“1”), and status of 6th to 8th bits. The switching request of the K1 byte is such that “1011” represents the above-mentioned SF-RING, “0001” represents the RR-RING and “0000” represents no request. If status represented by the K2 byte is “111”, this indicates an AIS (Alarm Indication Signal).
Squelch
Since the same channel can be used by multiple paths in a BLSR network, misconnection of paths occurs if failures develop at multiple locations. In order to prevent such misconnection, the P-AIS (Path Alarm Indication Signal) is inserted in the channel in which the misconnection occurred. This operation for inserting the P-AIS is referred to as “squelch”. A squelch table is used to execute squelch. The content of a squelch table specifies the add/drop node of each channel and is set in each node. As shown in FIG. 32A, a node has EAST and WEST sides. The direction in which a signal advances from the EAST to the WEST side through the node is referred to as the EW direction, and direction in which a signal advances from the WEST to the EAST side through the node is referred to as the WE direction. As shown in FIG. 32B, the squelch table describes add/drop nodes in the WE and EW directions with regard to each of the EAST and WEST sides of the node on a per-channel basis. The add node is entered in the source-office name field of the squelch table and the drop node is entered in the destination-office name field. Accordingly, on the presumption that communication is performed bi-directionally between nodes (A) and (E), between nodes (A) and (C) and between nodes (C) and (E), as shown in FIG. 33, squelch tables SQTL-A through SQTL-H of respective ones of the nodes (A) through (H) become as illustrated. It should be noted that these squelch tables have been created using the node IDs of nodes (A) to (H).
The squelch tables are used to determine whether signals on respective channels can be rescued by loop-back if failures develop at two or more locations in a ring. There is the possibility that a signal judged to be unrescuable based upon the result of the determination made by a squelch table will be output from the wrong node, namely a node different from that intended. Squelch is executed if occurrence of such a misconnection is likely. The node that executes squelch is a switching node, and it does so when failures occur at two or more locations in a ring. Squelch is not executed in the following cases:
(1) when failures have occurred at both ends of the local node (i.e., when the local node is isolated);
(2) when a failure has not occurred on either side of the local node (i.e., when the local node is not a switching node); and
(3) when bridging or switching is not actually being performed.
Reference will be had to FIG. 31 to describe squelch decision processing at node (E) in a case where failure has occurred between node (E) and (D) and between nodes (F) and (G) simultaneously. If squelch is not executed, a signal on channel ch. 1 from node (A) to node (E) is looped back to the protection channel ch. 25 by a bridging function at node (G), and the protection channel ch. 25 is looped back to the working channel ch. 1 by a switching function at node (D), thereby causing a misconnection in which the signal from node (A) to node (E) is transmitted to node (D). Further, a signal on channel ch. 1 from node (E) to node (C) is looped back to the protection channel ch. 25 by a bridging function at node (E), and the signal is looped back to the working channel ch. 1 by a switching function at node (F), thereby causing a misconnection in which the signal from node (E) to node (C) is transmitted to a lower-order group via node (E).
Accordingly, if multiple failures have occurred, (1) the locations of the failures are identified, (2) nodes at which signals will not arrive (so-called “signal non-arrival” nodes) owing to the failures are found from the ring topology, (3) reference is had to the squelch tables to determine whether the nodes that have been entered in these tables are nodes at which signals will not arrive, and (4) if a node is one at which a signal will not arrive, then squelch is executed.
Ring topology is the topology obtained by arraying the names of nodes that construct the ring clockwise in order starting from the node of interest. FIG. 34 illustrates ring topology RTG of node (E). It is ascertained from the faulty locations and ring topology RTG of FIG. 34 that nodes at which signals will not arrive from node (E) are the nodes of node IDs 9, 6, 4, 1, 14, 3. It is determined whether source and destination nodes that have been entered in a squelch table SQTL-E of node (E) match nodes at which signals will not arrive. Since it is found that node (C) of node ID 14 and node (A) of node ID 4 are nodes at which signals will not arrive, squelch is executed. In other words, squelch is executed at the switching nodes (D), (E), (F) and (G) by inserting P-AIS in each of the channel signals after bridging and after switching.
Construction of Ring Topology
FIGS. 35A, 35B are diagrams useful in describing the construction of a ring topology.
In a system in which four nodes (A) to (D) are connected by a ring transmission path RL, an identification number is assigned to each node, as shown in FIG. 35A. For example, 15, 3, 7 and 8 are assigned as the IDs of nodes (A), (B), (C) and (D), respectively. Next, as shown in FIG. 35B, (1) node (A), which specifies the construction of the ring topology (ring map), sends a ring topology frame RTGF, in which the inserted-node number is 1 and ID 15 of its own node is assigned to the first field. The ring topology frame RTGF is sent in the clockwise direction, by way of example. (2) Next, node (B) sends a ring topology frame RTGF, in which the inserted-node number is 2 and the ID of its own node is inserted following the ID of node (A). (3) Similarly, node (C) sends a ring topology frame RTGF, in which the inserted-node number is 3 and the ID of its own node is inserted following the ID of node (B), and (4) node (D) sends a ring topology frame RTGF, in which the inserted-node number is 4 and the ID of its own node is inserted following the ID of node (C).
(5) Since the first inserted-node ID is its own node ID, node (A) recognizes that the frame has come full circle and, as shown in FIG. 35C, transmits the ring topology frame RTGF upon inserting an END flag at the end thereof, whereby each node is notified of the completed ring topology frame. Each node that has received this ring topology frame constructs a ring topology with its own node at the head. For example, the ring topology is “15, 3, 7, 8” at node (A), “3, 7, 8, 15” at node (B), “7, 8, 15, 3” at node (C) and “8, 15, 3, 7” at node (D). Such a ring topology makes it easy to send a local node ID and a target node ID using the K1, K2 bytes in accordance with the APS protocol.
With conventional ring networks, a VT channel is assigned to a user permanently. Consequently, even if communication becomes impossible owing to occurrence of multiple failures and squelch is executed on a prescribed channel in accordance with the BLSR scheme, there is no effect upon the traffic of other channels that flows through the ring network. That is, even if squelch is carried out at a node where loop-back is in effect owing to failure and P-AIS is inserted into this channel, there is no influence upon the traffic of other channels.
However, in a ring network in which a VT channel is assigned to a user permanently, as in the prior art, the bandwidth (1.728-Mbps) of the user channel is not used unless the user communicates. This means that transmission band is not utilized effectively. According, there has been proposed a ring network in which communication is performed upon setting up an arbitrary connection dynamically for a transmission path which has an empty transmission band. This ring network transmits packets (IP packets, ATM cells, etc.) having various connection IDs upon mapping the packets into the payload of a POS (Packet Over Sonet or Packet Over SDH) frame.
However, a problem with this proposed ring network is that needless traffic flows through the ring when multiple failures occur. FIGS. 36A, 36B are diagrams useful in describing this problem of the proposed ring network, in which FIG. 36A is a diagram useful in describing the path of a packet (assumed to be ATM cells) in a ring network in which a failure has not occurred, and FIG. 36B is a diagram useful in describing a P-AIS route in a case where failures have occurred between nodes F and G and between nodes D and E. The dashed line in FIG. 36B indicates the route along which the P-AIS flows. Consider a case where squelch has been carried out and the P-AIS inserted by node G, at which loop-back is performed, in a manner similar to that of the BLSR scheme in conventional SONET.
An ATM cell that has been inserted from node A passes through node H and is looped back at node G. Owing to multiple failures, however, P-AIS is inserted and communication continues (as indicated by the dashed line). When squelch is executed at node G where loop-back is being performed, however, an ordinary ATM cell flows through the section of the ring (indicated by the solid line) from the insert node (node A) to the node (node G) performing loop-back. Since this ATM cell is discarded by squelch at the loop-back node, needless traffic TRF flows through in the ring.
A packet communication network based upon POS is characterized in that it assigns a transmission band in response to a connection request by a best-effort service when there is an empty transmission band, thereby making it possible to use transmission band effectively.